Principle characteristics of capacitive electromagnetic flowmeter

Embed the electrode in the lining so that the fluid to be measured does not contact the electrode. In this way, the problems of electrode corrosion, pollution, and liquid leakage can be fundamentally solved. For such a sensor, the flow signal is transmitted to the converter through the capacitance formed by the electrode and the lining. This is the capacitive electromagnetic flowmeter. In order to increase the capacity of the capacitor and reduce the signal loss on the capacitive reactance, the electrode is usually buried in the lining with a large surface so that the measured fluid does not contact the electrode. In this way, the problems of electrode corrosion, pollution, and liquid leakage can be fundamentally solved. For such a sensor, the flow signal is transmitted to the converter through the capacitance formed by the electrode and the lining. This is the capacitive electromagnetic flowmeter.　　In order to increase the capacity of the capacitor and reduce the signal loss on the capacitive reactance, a large-area electrode or a multi-electrode structure is usually used. The capacity of the coupling capacitor is only in the range of tens to thousands of picofarads. To ensure that most of the signal can be transmitted to the amplifier, the input resistance of the converter must be increased to more than 1012Ω. Therefore, the capacitive electromagnetic flowmeter has the advantage of being able to measure low-conductivity liquids. For example, when measuring pure water treated with deionized purified water, its conductivity may be below 10-5S/m. Using a general electromagnetic flowmeter according to the analysis in Section 4 of this chapter, the measurement error can reach more than 5%.　　The variation of the coupling capacitance will cause large noise, which will affect the accuracy and stability of the measurement. For this reason, the lining material used as a capacitor dielectric should have the properties of high insulation strength, low dielectric loss coefficient, low temperature coefficient, and good rigidity. The new lining material PFA (polytetrafluoroethylene) and high-purity alumina industrial ceramics can meet these requirements.　　 There are distributed capacitance and insulation resistance between the excitation coil and the electrode. The excitation voltage is divided by the distributed capacitance and insulation resistance between the excitation coil and the electrode and the internal impedance of the signal, which can form a large common mode interference voltage on the electrode. The signal internal resistance of the capacitive electromagnetic flowmeter is very high, and the common mode interference of the partial pressure may be greater. Therefore, improving the common-mode rejection ratio of the meter becomes another technical difficulty of the capacitive electromagnetic flowmeter. The common mode rejection ratio of the capacitive electromagnetic flowmeter is required to be CMRR≥120dB. Putting the preamplifier in the sensor, using bootstrap impedance conversion and electrostatic shielding measures as shown in Figure 2.23, can effectively suppress large common mode interference.　　Capacitive electromagnetic flowmeter can display the characteristics of stable measurement in solving solid-liquid two-phase slurry fluid measurement, and can really effectively solve the phenomenon of output swing caused by DC polarization interference. Capacitance transmission type flowmeter can measure the flow rate of low-conductivity fluid with high accuracy. However, it is necessary to pay attention to the problem of low conductivity'flow noise' mentioned above when using it. At this time, the flow rate range measured by the flowmeter should be shown in Figure 2.24, and the upper limit flow rate is within the available range. It should be noted that although the measuring electrode does not touch the measured fluid and there is no electrode corrosion problem, in order to obtain the differential flow signal, metal parts such as the wetted ring, the wetted electrode, and the wetted short tube are still required during installation. The corrosion problem of metal materials still exists.